6.7.1 EQUIPMENT AND SUPPLIES

Equipment and supplies commonly used for field measurement of turbidity are
listed in table 6.7-1. Before field
use of water-quality instruments, become familiar with the manufacturer's
instructions for calibration, operation, and maintenance. Test
field instruments before use.

Turbidity instruments. Three basic types of
instrumentation are used to measure turbidity: turbidimeters
(nephelometers), spectrophotometers, and multiparameter instruments with
submersible sondes that can accommodate a turbidity sensor (commonly
referred to as a turbidity probe). Choice of turbidity instrument depends
on site characteristics and intended use of the data in addition to
instrument specifications, performance, and reliability.2

If
measuring turbidity for regulatory or compliance purposes, the only method
approved by the USEPA employs Method 180.1 (STORET NO. 00076) (USEPA,
1979).3

For nonregulatory monitoring purposes, either a
submersible sensor that measures turbidity using a near-infrared light
source or a spectrophotometer in absorbance mode may be used.

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Turbidity probes (submersible sensors) are available for
multiparameter instruments with pH, temperature, conductivity, and other
sensors; this is convenient for monitoring turbidity along with other field
measurements. For ground-water studies, multiparameter instruments are
available with sondes that can be used in 2-in. diameter wells.

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Field spectrophotometers can be convenient for qualitative turbidity
measurements if additional sample properties will be measured
spectrophotometrically.

Selected turbidity instruments were tested by the Hydrologic
Instrumentation Facility. Referring to table 6.7-2, field tests of the
Hach DR 2000TM indicated consistently higher
FTU values compared with NTU values measured with the Analite 152TM, Hach 2100PTM,
Hydrolab DataSonde 3TM, and YSI 3800TM. Not available at the time of testing were
either the Hydrolab H20TM or YSI 6000TM multiparameter instruments with turbidity probe
or the Analite 156TM. Refer to Hydrologic
Instrumentation Facility (1994) for test details.

The method used for sample handling falls into three general categories, as
dictated by instrument capabilities: (1) manual (discrete) sample, using a
cuvette-based instrument, with sample decanted into a sample cell
(cuvette); (2) pumped sample, in which a sample is pumped through a
"flowthrough cell," which is a turbidity-sensor-containing cuvette that is
an internal part of the instrument; and (3) direct determination, by
positioning a turbidity probe either in situ or into a flowthrough chamber
that receives pumped sample (see NFM
6.0).

Turbidity-free water. Turbidity-free water is used for
preparation of turbidity standards and is prepared by filtering either
sample water or deionized water (DIW) through a 0.2-mm or smaller
pore-sized membrane. Turbidity-free water is recommended instead of
unfiltered DIW for preparation of standards.

Debubbler/degassing system. Bubbles in the sample will
give false turbidity readings. A debubbler or degassing system is required
if sample contains effervescing gases. The equipment plumbing must be set
up to maintain a constant head, resulting in constant velocity through the
turbidimeter's flowthrough cell. When using a turbidity probe within a
flowthrough chamber, it might be necessary to direct debubbled water
through the chamber.

Probe-based instruments are available with a wiper
mechanism that clears bubbles from the optical surface of the submersible
sensor (probe).

Instruments with gas-sweep capacity. Condensation must
be removed or reduced throughout turbidity determination. Some
flowthrough-cell instruments have the capacity to continuously sweep the
sample compartment with dry gas, reducing condensation on the sample cell;
otherwise, condensation is to be removed manually every few minutes.

Check manufacturer's instructions for instrument maintenance, cleaning, and
storage. Test equipment before each field trip and record all
repairs in the instrument log book. Manufacturer's instructions
and the log book should accompany the instrument at all times.

Turbidity instruments. Protect instruments from extreme
temperatures. Shield the instrument LED display panel from direct
sunlight. If a bench-top turbidimeter gets wet, allow it to dry thoroughly
before the next use (field turbidimeters are constructed to withstand
moisture). Check and replace batteries routinely.

Sample cells (cuvettes). Handle and store sample cells in
a manner to prevent dirt, scratches, or other damage. Follow instrument
manufacturer instructions for the maintenance of sample cells. Keep sample
cells scrupulously clean, inside and out. After each use, (1) wash with
nonphosphate laboratory detergent, (2) rinse repeatedly with deionized
water until all detergent residue is removed, and (3) allow cells to air
dry in a dust-free environment.

Submersible turbidity probe. Exercise care that optical
surfaces of probes are not scratched during cleaning, operation, or
storage. Scratched or damaged probes must be replaced. Keep optical
surfaces free of all foreign material by wiping with moist lens-cleaning
paper or cloth.

Standard solutions. Discard turbidity standards with
elapsed expiration dates. Protect turbidity standards from extreme
temperatures. Never pour used standard or a portion of unused standard
back into its original (stock) container.

2Turbidity instruments are being developed and improved by
several companies; investigate instrument performance and reliability
before making an equipment selection.

3The USEPA also approves the GLI-2 method turbidity
instrument system (a microprocessor-based turbidity system using a
pulsed-light, four-beam sensor); the GLI-2 provides stable and reproducible
turbidity readings to 0.5 NTU but it is not a portable
instrument.